There is an increasing use of servo-controlled gearboxes, which are structurally similar to manual gearboxes of the traditional type, except for the fact that the clutch pedal and the gear selection lever operated by the user are replaced by corresponding electrical or hydraulic servo-controls. When using a servo-controlled gearbox, the user only needs to send the order to shift up or down to a transmission control unit and the transmission control unit autonomously shifts by acting both on the engine and on the servo-controls associated to clutch and gearbox.
The gear shifting order may be generated either manually, i.e. following a command imparted by the driver, or automatically, i.e. regardless of the action of the driver. When the gear shifting order is generated, the transmission control unit drives the servo-control associated to the clutch to open the clutch so as to mechanically separate a primary shaft of the gearbox from a crankshaft; at the same time, the transmission control unit acts on the engine control unit to temporarily reduce the motive torque output by the engine itself.
Once the transmission control unit has checked opening of the clutch, the transmission control unit drives the servo-controls associated to the clutch to disengage the currently engaged gear; when the transmission control unit has checked gear disengagement, the transmission control unit drives the servo-controls associated to the gearbox to shift the primary shaft with respect to a secondary shaft so as to arrange engagement of the new gear. Once the transmission control unit has checked that the primary shaft has reached the required position with respect to the secondary shaft, the transmission control unit drives the servo-controls associated to the gearbox to engage the new gear.
Finally, when the transmission control unit has checked that the new gear has been engaged, the transmission control unit drives the servo-control associated to the clutch to close the clutch so as to make the primary shaft of the gearbox and the crankshaft reciprocally and angularly integral; at the same time, the transmission control unit acts on the engine control unit to restore the motive torque of the engine itself.
In normal driving conditions, the servo-controlled gearbox is required to rapidly shift gears without triggering abrupt longitudinal acceleration/deceleration on the vehicle, which are annoying for vehicle passengers and subject the vehicle transmission to unnecessary mechanical strain. In sporty driving conditions, the servo-controlled gearbox is required to shift gears as fast as possible without subjecting the transmission to excessive mechanical strain which could lead to damaging the transmission itself.
The currently marketed servo-assisted gearboxes operating according to the above-described method require a time generally from 250 to 600 ms to shift a gear; the time actually employed depends both on the dynamic performance of the gearbox components and on the required level of comfort. A 250 ms gear shift is already very fast; however, it is still relatively slow for sporty driving, especially for competitive track racing.
It is important to observe that the gear must be shifted, compatibly with requirements of comfort and mechanical protection, as rapidly as possible during gear progression, i.e. when shifting from a lower gear to a higher gear, because during gear progression the engine is ‘driving’ to accelerate the vehicle and consequently must be separated from the drive wheels for the shortest possible time; instead, when shifting down, i.e. when shifting from a higher gear to a lower gear, the gear shift may also be slower, because vehicle deceleration is essentially performed by the braking system and the engine does not have an essential role in vehicle dynamics.
It is known that gear shifting time is mainly determined by the new gear synchronisation time, i.e. by the time employed by the synchronisers to adapt the angular velocity of the primary shaft of the gearbox to the angular velocity determined by the new ratio.
In order to reduce the synchronisation time during gear progression, i.e. when shifting from a lower gear to a higher gear, the use of a braking device coupled to the primary shaft of the gearbox has been proposed so as to brake the primary shaft itself and rapidly adapt the angular velocity of the primary shaft to the angular velocity determined by the new ratio. However, this solution is relatively costly and complicated due to the need of arranging and controlling a brake coupled to the primary gearbox shaft.
Furthermore, as described in patent application EP1201483A2, in order to reduce the synchronisation time during gear progression, a method of disengaging gears in a servo-controlled gearbox has been proposed according to which an oscillation is generated on the angular velocity of a primary shaft of the gearbox by abruptly opening the respective clutch, and the gear is disengaged about the maximum amplitude of a first oscillation half-wave, when the oscillation itself has taken the angular velocity of the primary shaft close to the angular velocity that the primary shaft must assume to engage the next gear. However, also the method proposed by patent application EP1201483, while reducing the required shifting time, does not allow to reach the extremely short shifting times required by sporty driving.